1. Field of the Invention
The present invention generally relates to a battery charge control circuit, a battery charging device, and a battery charge control method.
Charging a lithium-ion secondary battery is performed by a constant voltage/current control circuit, and the completion of the charging operation is normally determined when the charging current for the battery becomes smaller than a predetermined reference value.
In the case where completion of the charging operation is determined when the charging current value becomes smaller than the predetermined reference value, a charging device is expected to constantly supply a charging current larger than the predetermined reference value. However, when the battery is charged by a charger contained in an electronic device such as a notebook computer, only the difference between the power supply capacity of an AC adapter and the power consumption of the notebook computer is available to supply the charging current. In such a case, the charging current required by the battery is not always supplied to the battery.
When the charging current for the secondary battery becomes extremely small due to high power consumption by the notebook computer, a wrong determination that the charging operation has been completed is made. To avoid such an error, a charging constant voltage/current control circuit outputs a signal to determine whether the charging current is limited because the load of the electronic device is heavy or because the battery is actually fully charged.
In a portable electronic device such as a notebook computer, a battery is mounted as a power source for the electronic device. Generally, such a battery is a lithium battery in consideration of operating costs and instantaneously dischargeable current capacity. Also, a charger circuit is often contained in a portable electronic device, so that a secondary battery in the electronic device can be readily charged simply by connecting an AC adapter to the electronic device. For its portability, a portable electronic device normally has an internal secondary battery as a power source. However, when used on a desk, it might be supplied with power from an external power source such as an AC adapter.
A lithium secondary battery often used in notebook computers is charged at a constant voltage and/or a constant current. And the completion of the charging operation is normally determined when the charging current value becomes smaller than a predetermined reference value.
There are various techniques for charging a secondary battery by a charger contained in an electronic device such as a notebook computer. For example, the secondary battery is charged with power supplied from an external device such as an AC adapter, and the charging operation is performed whether or not the electronic device is in operation.
2. Description of the Related Art
FIG. 1 is a block diagram showing the structure of a conventional power supply unit for notebook (or lap-top, portable) computers.
An AC adapter 1 is connected to an AC power supply 2, and converts alternating current supplied from the AC power supply 2 into direct current. The AC adapter 1 is also connected to a power supply connector 3. The power supply connector 3 is in turn connected to a DC/DC converter 4 via a resistor R1 and a diode D1. The DC/DC converter 4 is connected to a secondary battery 5 via a diode D2, and converts DC power supplied from the AC adapter 1 or the secondary battery 5 into a predetermined DC voltage to be supplied to a load 6.
The secondary battery 5 is connected to a charger circuit 24 which comprises a voltage/current regulator 8, a differential amplifier 9, a voltage comparator 10, reference voltage supplies 12 to 14, and a microcomputer (or microprocessor) 11.
The voltage/current regulator 8 is a switching regulator-type DC/DC converter that operates in a PWM control system. The voltage/current regulator 8 comprises a switching transistor Tr1, a choke coil L1, a flywheel diode D3, a smoothing capacitor C1, a charging current detecting resistor R0, and a control unit 7.
The switching transistor Tr1 is formed by an FET, and is switched on and off by the control unit 7. The charging current detecting resistor R0 is a sense resistor which measures the value of current for charging the battery 5. A voltage drop caused by the current flowing through the sense resistor is inputted into the control unit 7. The switching transistor Tr1 is switched on and off to control current flowing through the choke coil L1. Thus, the voltage/current regulator 8 can perform DC/DC control.
Both ends of the charging current detecting resistor R0 are connected to the differential amplifier 9.
The non-inverting input terminal of the differential amplifier 9 is connected to the connection point between the charging current detecting resistor R0 and the battery 5, while the inserting input terminal of the differential amplifier 9 is connected to the connection point between the charging current detecting resistor R0 and the choke coil L1. The differential amplifier 9 amplifies voltages at both ends of the charging current detecting resistor R0. The output of the differential amplifier 9 is a voltage corresponding to the current supplied to the battery 5. The output of the differential amplifier 9 is supplied to the microcomputer 11.
The non-inverting input terminal of the voltage comparator 10 is connected to the AC adapter 1, and the inverting input terminal of the voltage comparator 10 is connected to the reference voltage supply 12. The voltage comparator 10 outputs a high-level signal or a low-level signal depending on the voltage of the AC adapter 1. More specifically, when the voltage generated from the AC adapter 1 is higher than a reference voltage supplied from the reference voltage supply 12, the voltage comparator 10 outputs a high-level signal. When the voltage generated from the AC adapter 1 is lower than the reference voltage supplied from the reference voltage supply 12, the voltage comparator 10 outputs a low-level signal. When the AC adapter 1 is connected to the charger circuit 24, the voltage comparator 10 outputs the high-level signal. When the AC adapter 1 is not connected, the voltage comparator 10 outputs the low-level signal. The output signals of the voltage comparator 10 are supplied to the microcomputer 11.
The microcomputer 11 controls the operation of the control unit 7 in accordance with the output signals of the differential amplifier 9 and the voltage comparator 10. When the output of the differential amplifier 9 is higher than a predetermined voltage, i.e., when the charging current is flowing, the microcomputer 11 determines that the battery 5 is not fully charged. When the output signal of the voltage comparator 10 is high, the microcomputer 11 determines that the AC adapter 1 is connected to the charger circuit 24.
After determining that the battery 5 and the AC adapter 1 are connected from the outputs of the differential amplifier 9 and the voltage comparator 10, the microcomputer 11 determines that the battery 5 can be charged, and supplies a control signal to the control unit 7 to switch on the control unit 7. When the output of the differential amplifier 9 is lower than the predetermined voltage, i.e., when the battery 5 is in a fully charged state, or when the output signal from the voltage comparator 10 is low, i.e., when the AC adapter 1 is not connected to the charger circuit 24, the microcomputer 11 determines that the battery 5 cannot be charged any more, and supplies a control signal to the control unit 7 to switch off the control unit 7.
Besides the control signals from the microcomputer 11, the control unit 7 receives the voltages from both ends of the resistor R1, the voltages from both ends of the charging current detecting resistor R0, and reference voltages. The control unit 7 is controlled in accordance with the control signals from the microcomputer 11, and switches on and off the switching transistor Tr1 in accordance with the voltages from both ends of the resistor R1, the voltages from both ends of the charging current detecting resistor R0, and the reference voltages.
The circuit shown in FIG. 1 charges the battery 5 by the charger circuit 24 while supplying power to the load 6. The input from the AC adapter 1 is supplied to the battery 5 through the charger circuit 24 as well as to the load 6 through the DC/DC converter 4. Accordingly, the load 6 consumes power while the battery 5 is charged.
FIG. 2 is a block diagram of the control unit of the conventional power supply unit.
The control unit 7 comprises differential amplifiers 15 and 16, error amplifiers 17 to 19, a triangular wave oscillator 20, a PWM comparator 21 and a driver 22.
The differential amplifier 15 detects the voltages at both ends of the resistor R1. The output of the differential amplifier 15 turns into a signal corresponding to the current flowing through the resistor R1, i.e., to the output current of the AC adapter 1.
The differential amplifier 16 detects the voltages at both ends of the charging current detecting resistor R0. The output of the differential amplifier 16 turns into a signal corresponding to the current flowing through the charging current detecting resistor R0, i.e., to the charging current for charging the battery 5.
The output detection signal from the differential amplifier 15 is supplied to the inverting input terminal of the error amplifier 17. A reference voltage Vref1 from a reference voltage supply 13 is applied to the non-inverting input terminal of the error amplifier 17. The error amplifier 17 in turn outputs a signal corresponding to the difference between the output from the differential amplifier 15 and the reference voltage Vref1. The reference voltage Vref1 is set in accordance with the maximum current supplied from the AC adapter 1.
The output detection signal from the differential amplifier 16 is supplied to the non-inverting input terminal of the error amplifier 18. A reference voltage Vref2 from a reference voltage supply 14 is applied to the inverting input terminal of the error amplifier 18. The error amplifier 18 in turn outputs a signal corresponding to the difference between the output from the differential amplifier 16 and the reference voltage Vref2.
The inverting input terminal of the error amplifier 19 is connected to the connection point between the charging current detecting resistor R0 and the battery 5, and the non-inverting input terminal is connected to a reference voltage supply 23. The error amplifier 19 outputs the difference between the reference voltage Vref3 from the reference voltage supply 23 and the charging voltage for the battery 5 at the connection point between the charging current detecting resistor R0 and the battery 5. The output of the error amplifier 19 is supplied to the PWM comparator 21. The reference voltage Vref3 is set in accordance with the maximum voltage applicable to the battery 5.
The triangular wave oscillator 20 outputs a signal whose output level shows a saw-tooth waveform. The signal generated from the triangular wave oscillator 20 is supplied to the PWM comparator 21.
The PWM comparator 21 compares the respective outputs of the error amplifiers 17 to 19 with the saw-tooth wave signal generated from the triangular wave oscillator 20. In accordance with the comparison results, the PWM comparator 21 generates a high-level signal or a low-level signal, and outputs a pulse according to the AND logic. The output pulse of the PWM comparator 21 is supplied to the driver 22. In accordance with the output pulse, the driver 22 switches on and off the switching transistor TR1.
FIG. 3A shows a triangular waveform of the outputs of the error amplifiers 17 to 19. FIG. 3B shows the switching state of the switching transistor Tr1.
As shown in FIG. 3A, the PWM comparator 21 compares the minimum voltage level among the outputs of the error amplifiers 17 to 19 with the saw-tooth wave supplied from the triangular wave oscillator 20. When the minimum voltage level among the outputs of the error amplifiers 17 to 19 is higher than the saw-tooth wave supplied from the triangular wave oscillator 20, the switching transistor Tr1 is switched on, as shown in FIG. 3B. The switching transistor Tr1 is switched off during the other periods.
Being switched on and off, the switching transistor Tr1 outputs a pulse-type current. The current outputted from the switching transistor Tr1 is rectified by the rectifier circuit, and is supplied to the battery 5. The voltage and current supplied to the battery 5 here is controlled by the ON/OFF periods of the switching transistor Tr1. Such a control operation is called xe2x80x9cPWM controlxe2x80x9d.
The error amplifier 17 shown in FIG. 2 amplifies the difference between the output of the differential amplifier 15 and DC-CURR (the reference voltage Vref1) supplied from the reference voltage supply 13 shown in FIG. 1. As mentioned before, the DC-CURR (the reference voltage Vref1) supplied from the reference voltage 13 shown in FIG. 1 is set in accordance with the maximum current value the AC adapter 1 can supply. Accordingly, the output of the error amplifier 17 activates the driver 22 through the PWM comparator 21, so that the sum of the currents that the AC adapter 1 supplies to the load 6 and the battery 5 equals the maximum current the AC adapter 1 can supply.
While the power is supplied from the AC adapter 1 to the load 6, the error amplifier 17 increases and decreases the charging current for the battery 5 as the power consumption by the load 6 increases and decreases. By doing so, the error amplifier 17 controls the charging current so that the sum of the current consumed by the load 6 and the charging current for the battery 5 equals the maximum power capacity of the AC adapter 1. For instance, when the current consumption of the load 6 increases, the current flowing through the sense resistor R1 also increases. As the current flowing through the sense resistor R1 increases, the output of the differential amplifier 15 becomes larger. As the output of the error amplifier 15 becomes larger, the difference between the output of the error amplifier 15 and the DC-CURR (the reference voltage Vref1) supplied from the reference voltage supply 13 becomes small, and so does the output of the error amplifier 17. When the output of the error amplifier 17 becomes smaller than the outputs of the error amplifiers 18 and 19, the PWM comparator 1 compares the output of the error amplifier 17 with the output of the triangular wave oscillator 20. In accordance with the comparison result between the outputs of the error amplifier 17 and the triangular wave oscillator 20, the PWM comparator 21 drives the driver 22.
When the current consumption of the load 6 increases, the output of the error amplifier 17 is smaller than the outputs of the error amplifiers 18 and 19. Accordingly, the error amplifier 17 is controlled to restrict the charging current for the battery 5.
The output of the differential amplifier 16 corresponding to the current flowing through the sense resistor R0 shown in FIG. 1 and the reference voltage Vref2 (BAT CURR) outputted from the reference voltage supply 14 define the maximum charging current that can be applied to the battery 5. Accordingly, the output of the error amplifier 18 serves to maintain the charging current for the battery 5 at a predetermined current value.
The error amplifier 19 amplifies the difference between the charging voltage ERR2 for the battery 5 and the reference voltage Vref3 generated from the reference voltage supply 23. The reference voltage Vref3 generated from the reference voltage supply 23 is set in accordance with the maximum voltage that can be applied to the battery 5. Accordingly, the output of the error amplifier 19 serves to activate the driver 22 so that the battery 5 has the maximum voltage level.
As mentioned before, the outputs of the error amplifiers 17 to 19 are inputted into the non-inverting input terminal of the PWM comparator 21. The minimum voltage level of the error amplifiers 17 to 19 is used to control the switching transistor Tr1. More specifically, when the output of the error amplifier 18 is at the minimum voltage level, the switching transistor Tr1 is switched on and off so as to turn the power to be supplied to the battery 5 into a constant current. In the field of DC/DC conversion, a circuit for controlling a charging current so as to be a constant current is called a current regulator, a constant-current control circuit, or a constant-current charger control circuit. This constant-current charging will be described later in detail, with reference to FIG. 5.
When the output of the error amplifier 19 is at the maximum voltage level, the voltage to be applied to the battery 5 is a constant voltage. Accordingly, the circuit for turning the charging voltage into a constant voltage is called a constant-voltage circuit, a voltage regulator, a constant-voltage control circuit, or a constant-voltage charger control circuit. This constant-voltage charging will be described later in detail, with reference to FIG. 5.
A circuit having both a current regulator and a voltage regulator or both functions of a current regulator and a voltage regulator is called a constant voltage/current control circuit or a voltage/current regulator.
FIG. 4 is a flowchart of an operation of the microcomputer of a conventional power supply unit.
First in step S1-1, the microcomputer 11 determines whether all charge starting conditions are satisfied or not. The charge starting conditions that represented by voltages are: that the AC adapter 1 is supplying a voltage, that the battery 5 is connected, and that the battery 5 is not full.
When the output of the voltage comparator 10 is high, the microcomputer 11 determines that a voltage is supplied from the AC adapter 1. By detecting whether the output of the differential amplifier 9 is higher than a predetermined level or not, the microcomputer 11 determines whether the battery 5 is fully charged or not. When the battery 5 is not fully charged, a current flows through the charging current detecting resistor R0, generating voltages at both ends of the charging current detecting resistor R0, and making the output of the differential amplifier 9 higher than the predetermined level.
When all the charge starting conditions are satisfied, the microcomputer switches on the control unit 7 in step S1-2. In accordance with the voltages at both ends of the resistor R1 and the charging current detecting resistor R0, the control unit 7 performs PWM control on the current to be supplied to the battery 5.
In step S1-3, the microcomputer 11 determines whether the charging current becomes lower than a predetermined value during the charging. This determination is made based on the output signal from the differential amplifier 9. When the charging current becomes lower than a predetermined value, the voltages at both ends of the charging current detecting resistor R0 drop, and the output of the differential amplifier 9 becomes small. Thus, whether the charging current is lower than the predetermined value can be determined from the output of the differential amplifier 9.
If the charging current is determined not to be smaller than the predetermined value in the step S1-3, the charging is continued. If the charging current is determined to be smaller than the predetermined value in the step S1-3, the 93 microcomputer 11 determines that the charging of the battery 5 has been completed, and stops the operation of the control unit 7, thereby stopping the charging of the battery 5.
FIG. 5A shows the charging voltage characteristics of the battery 5, and FIG. 5B shows the charging current characteristics of the battery 5.
As shown in FIG. 5A, if the battery 5 is in a constant-voltage state at time t1, the charging current I decreases after the time t1 as shown in FIG. 5B. When the charging current I reaches a predetermined level I0 at time t2, as shown in FIG. 5B, the microcomputer 11 stops the operation of the control unit 7, thereby stopping the charging of the battery 5.
More specifically, when the current flowing through the load 6 is not large, the control unit 7 controls the charging by the output of either the error amplifier 18 or the error amplifier 19, because the output of the error amplifier 17 does not become the smallest one among the three error amplifiers 17 to 19. In FIGS. 5A and 5B, at the start of charging the battery 5 (a lithium battery, specifically), the output of the error amplifier 18 is smaller than the other positive inputs. Therefore, the control unit 7 controls the charging current so that the battery 5 is charged with a constant current until the time t1, as shown in FIG. 5B. Accordingly, in the initial stage of charging, the error amplifier 18 provides the battery 5 with a current having a value corresponding to the reference voltage Vref2 generated from the reference voltage supply 14.
As shown in FIG. 5A, when the voltages rises to a predetermined voltage at the time t1, the output voltage of the error amplifier 19 shown in FIG. 2 becomes the lowest, and the charging is controlled with the output of the error amplifier 19. After the time t1, the voltage to be applied to the battery 5 is controlled to be a constant voltage. As mentioned before, the charging current gradually decreases after the time t1.
It should be noted that Japanese Laid-Open Patent Application No. 8-182219 discloses a battery charge control circuit having the above structure.
In the conventional charger circuit, however, the switching transistor Tr1 is controlled by the control unit 7 in accordance with the voltage of the AC adapter 1 and the current to be supplied to the battery 5. When the current demanded by the load 6 increases and exceeds the current supply capacity of the AC adapter 1, most of the output current of the AC adapter 1 is supplied to the load 6 through the resistor R1, the diode D1, and the DC/DC converter 4, even though the battery 5 is not fully charged.
The AC adapter is connected to the battery 5 as well as to the load 6. The battery 5 can be charged even when the load 6 is on (i.e., when the load 6 consumes power). Accordingly, the AC adapter 1 charges the battery 5 and supplies the load 6 with power at the same time. When the power consumption of the load 6 is not very large, the battery 5 is charged in accordance with the charging characteristic shown in FIGS. 5A and 5B. If the power consumption of the load 6 becomes larger than the current supply capacity of the AC adapter 1, the switching transistor Tr1 is controlled in accordance with the output of the error amplifier 17 shown in FIG. 2, and the charger 6 is supplied with less and less current. This is because the error amplifier 17 drives the driver 22 via the PWM comparator 21, so that the sum of the currents to be supplied to the load 6 and the battery 5 equals the maximum supply current of the AC adapter 1. Accordingly, while the load 6 is supplied with the power from the AC adapter 1, the error amplifier 17 supplies current to the load 6 in accordance with the power consumption of the load 6. Accordingly, if the power consumption of the load 6 becomes equal to the maximum supply current of the AC adapter 1, the charger circuit 6 receives no current at all, and no current flows through the charging current detecting resistor R0. As no current flows through the charging current detecting resistor R0, the voltage of the charging current detecting resistor R0 drops. When the voltage of the charging current detecting resistor R0 drops, the microcomputer 11 determines that the charging of the battery 5 has been completed, and stops the operation of the control unit 7.
The above wrong determination is likely to occur when the capacity of the AC adapter is not sufficiently large.
In a case where a plurality of secondary batteries are mounted in an electronic device such as a notebook computer, one charger circuit charges the plurality of secondary batteries connected in parallel. In such a parallel charging operation, more charging current flows into batteries having less power left than the other batteries, while less or no charging current flows into the other batteries having more power left. If one of the batteries has only an extremely small amount of power left, the remaining batteries might be supplied with no power at all. With no power being supplied, the microcomputer might wrongly determine that the charging has been completed.
As described above, the conventional charger circuit has the problem that the operation of the control unit 7 is stopped even though the battery 5 is not fully charged.
Also, as mentioned before, when a battery is charged by a charger for an electronic device such as a notebook computer, the required amount of current may not always be supplied to the secondary battery, in an attempt to perform the charging in a shortest possible period of time. If the electronic device requires a large amount of power to operate, the charging current to be supplied to the secondary battery becomes very small. As a result, the wrong determination that the charging of the secondary battery has been completed will be made.
A general object of the present invention is to provide battery charge control circuits, battery charging devices, and battery charge control methods, in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a battery charge control method, in which a wrong determination as to whether the charging of a battery has been completed can be prevented. Another specific object of the present invention is to provide a battery charge control circuit, a battery charging device, and a battery charge control method, in which wrong operations of a charger circuit can be prevented.
The above objects of the present invention are achieved by a battery charge control circuit, which has a restricted state notifying unit which detects a restriction on the supply capacity of a power source, and outputs a notification that the supply capacity of the power source is restricted.
With the above structure, a wrong determination as to whether the charging of a battery has been completed can be prevented, in a case where the supply capacity of the power source is restricted, a current is supplied to a load, and the charging current for the battery decreases accordingly.
The above objects of the present invention are also achieved by a battery charge control circuit, which includes a first control circuit for controlling the charging current for the battery so that the battery can be charged in accordance with predetermined charging conditions, and a second control circuit for controlling the charging current so that the power demanded from the power source does not exceed the capacity of the power source. In this battery charge control circuit, a notification when the charging current is being controlled by the second control circuit is outputted.
With the above structure, it can be determined that the supply capacity of the power source is restricted when the charging current is controlled by the second control circuit. Thus, no mistaken determination that the charging of the battery has been completed will be made when a current is supplied to a load and the charging current for the battery decreases accordingly.
The above objects of the present invention are also achieved by a battery charging device, which has a restricted state notifying unit which detects a restriction on the supply capacity of a power source, and outputs a notification that the supply capacity of the power source is restricted. The battery charging device may includes a first control circuit which controls the charging current of the battery so that the battery is charged in accordance with predetermined charging conditions, and a second control circuit which controls the charging current so that the power demanded from the power source does not exceed the capacity of the power source. In this battery charging device, the restricted state notifying unit outputs a notification that the charging current is controlled by the second control circuit.
The above objects of the present invention are also achieved by a battery charge control method comprising the steps of: detecting a restriction on the supply capacity of a power source which supplies current to a load and charges a battery part; and continuing the charging of the battery part when the supply capacity of the power source is restricted.
The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.